1. Field of the Invention
The present invention relates generally to improving the fabrication of integrated circuits and, more particularly, to reducing thickness-to-planarity and dishing during chemical mechanical planarization of layers on a substrate.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Microprocessors and memory devices, such as dynamic and static random access memories (DRAM and SRAM), are complex integrated circuits that are used in a wide variety of applications throughout the world. Such applications include personal computers, control systems, telephone networks, and a host of other consumer products. Despite their complexity, price competition requires that microprocessor and memory designs be inexpensive to manufacture. In the fabrication of integrated circuits, it can be a significant cost advantage to reduce the amount of material used and the waste generated.
Integrated circuits, such as random access memories, are fabricated on semiconductor wafers. They may be mass produced by fabricating thousands of identical circuit patterns on a single semiconductor wafer and subsequently dividing them into identical dies or chips. Integrated circuits are commonly referred to as “semiconductor devices” because the wafer substrates are typically a semiconductor, such as silicon. Integrated circuits are fabricated, however, by depositing numerous materials of varying electrical properties on the semiconductor wafer. These materials include insulators or dielectrics (such as silicon dioxide), conductors (such as polysilicon, copper, aluminum and tungsten), and semiconductors (such as silicon and germanium).
To produce the integrated circuit, many commonly known processes are used to modify, remove, and deposit the materials onto the semiconductor wafer. Processes such as ion implantation, sputtering, etching, chemical vapor deposition and variations thereof, such as plasma enhanced chemical vapor deposition, are among those commonly used. These steps of material deposition or removal on a semiconductor wafer are frequently followed by a planarization step such as chemical mechanical planarization (CMP).
Generally speaking, planarization is a process of removing material to render a surface relatively smooth. CMP, in particular, is the process of smoothing and planarizing aided by chemical and mechanical forces. The CMP process helps to minimize barriers to multilayer formation and metallization, as well as to smooth, flatten, and clean the surface. This process involves chemically modifying the surface while also mechanically polishing it. The combined action of surface chemical reaction and mechanical polishing allows for controlled layer-by-layer removal of the desired material from the wafer surface, resulting in the preferential removal of protruding surface topography and producing a planarized wafer surface. The degree of planarization may be measured, for example, with an optical reflectivity sensor. In the past few years, CMP has become one of the most effective techniques for planarizing a semiconductor wafer.
In general, the CMP process involves holding a semiconductor substrate, such as a wafer, against a rotating wetted polishing pad under controlled downward pressure. Alternately, the CMP process may involve holding a wetted polishing pad while rotating a semiconductor substrate, such as a wafer, under controlled downward pressure. A polishing slurry delivered onto the polishing pad contains etchants and an abrasive material such as alumina or silica. A wafer carrier is typically utilized to hold the wafer under controlled pressure against the polishing pad. The polishing pad may be constructed, for example, of a felt fabric material impregnated with blown polyurethane. To summarize, the three key elements in the CMP process include the surface to be polished, the pad which enables the transfer of mechanical forces to the surface being polished, and the slurry which provides both chemical and mechanical effects.
Abrasive particles in the slurry cause mechanical alteration at the sample surface, loosening the material for enhanced chemical attack or fracturing off the pieces of surface into a slurry where they dissolve or are swept away. For efficient planarization, it is typically desirable to enhance material removal rate from elevated regions or high points on the surface and to reduce material removal in recessed regions. Moreover, it is the combination of chemical and mechanical effects, and not these forces alone, that is typically advantageous. For example, chemistry alone typically will not achieve planarization because most chemical actions are isotropic. Mechanical grinding alone theoretically may achieve the desired planarization but is generally not desirable because of the extensive associated damage of the material surfaces.
To reduce the amount of material removed during CMP and thus the amount of material that must be deposited prior to CMP, it is typically preferred in CMP to remove material only from elevated regions on the sample surface. By removing material primarily from high points on the surface to achieve the specified degree of planarization, the amount of material removed, the related thickness-to-planarity, and the associated dishing phenomenon (discussed below) may be reduced. A problem in CMP, however, is that material is undesirably removed from recessed regions, increasing thickness-to-planarity and dishing. For example, during CMP the pad may deform and grind the recessed regions. As a result, in order to reach planarization, more material must be added initially to the wafer to account for the more material that is being removed from the recessed regions during CMP. Furthermore, even where the pad does not touch the recessed regions, the CMP slurry may etch material in the recessed regions. Again, more material must be removed to achieve planarity. More waste is generated and more material must be deposited initially to accommodate the significant amount of material removed during CMP.
Additionally, dishing, an undesirable effect occurs in filled contact openings. The recessed surface of the filled contact opening (“contact plug”) may be attacked by the slurry. Material is dissolved below the CMP stopping layer, forming a dish-shaped cavity on the wafer surface. Dishing may be characterized as the distance between the CMP stopping layer on the field and the contact plug surface. Significant material removal may occur in the recessed contact plug.
For metals, efforts to reduce dishing and thickness-to-planarity include the use of CMP slurries that not only dissolve the metal layer but also oxidize the metal surface or coat the metal surface with a polymer during CMP. Mechanical action removes the oxide or the polymer layer from the elevated regions (due to the higher pressure experienced), exposing the underlying metal to the slurry that can dissolve the metal, while the oxide or the polymer coating in the recessed areas may stay intact protecting the metal from the slurry.
For slurries that oxidize, a drawback is that the oxide layer formed on the metal surface is thin and relatively sparse, and hence, easily removed in the recessed regions due to CMP pad deformation. As a result, dishing and the thickness-to-planarity remain relatively high. For slurries that form polymers on the metal surface, the polymeric coating may protect the recessed areas better than a thin oxide layer but the coating is difficult to remove. Removal of a polymeric coating typically requires, for example, that the wafer be heated to a high temperature such as 100° C. Moreover, whether the slurry oxidizes the metal surface or coats the metal surface with polymer, these slurries are relatively expensive and do not significantly reduce thickness-to-planarity or dishing.
Other efforts to improve dishing characteristics involve the use of a harder CMP pad that deforms less than the typical CMP softer pads. Harder pads, however, can scratch wafers. Also, as will be appreciated by those of ordinary skill in the art, it is difficult to condition harder pads. For example, the use of a typical diamond-based conditioner may not effectively remove damaged portions of harder pads.
The present invention may be directed to one or more of the problems set forth above.